Generally, the OFDM scheme has attracted considerable attention in the wireless communication system field since it is appropriate for multi-path fading channels, and has high bandwidth efficiency. In the OFDM wireless communication system, a receiver functions to receive signals through an antenna, to demodulate the signals through an RF port and an IF port, and to recover the signals. In detail, in order to obtain desired signals, the RF port and the IF port receive the signals from the antenna, and then perform a frequency down-converting function and a signal-amplifying function for the signals. For this, the RF unit and the IF unit uses various analog elements including a mixer and an amplifier. The elements satisfy predetermined standards to a certain degree, but the elements are incompletely insulated from each other and have insignificant quadrature. Thus, there is a problem that the input signals are distorted. Particularly, the input signals are distorted by a DC offset. The DC offset is generated at the received original signal when drift signals are self-mixing in a quadrature converter. The drift signals are so-called abnormal signals occurring from an analogue circuit of an OFDM wireless communication system. In detail, the DC offset is generated at the received original signal when the RF port receives the mixed signals, which are the abnormal drift signals mixed at baseband, and an analogue-to-digital converter (ADC) converts the mixed signals into digital signals.
Recently, the direct-conversion method has been developed to produce a receiver at a low price. In the direct-conversion method, the received radio-frequency (RF) is down-converted directly to a baseband frequency without going through any intermediate frequency (IF).
By using the direct-conversion method, a receiver can directly convert the received RF signal to baseband so that many intermediate filters, mixers, and amplifiers may be omitted or simplified.
However, the direct conversion receiver has a problem of generating a large DC offset, since the direct-conversion receiver utilizes a local oscillator and the frequency of the local oscillator is approximately the same as the frequency of the received RF signal. Thus, the direct conversion receiver is keenly desired to remove a DC offset.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore, it may contain information that does not form the prior art that is already known in this country to a person or ordinary skill in the art.